I thought it was kind of weird that the noise was constant in all channels. Definitely took away the magic of the pre-amplier. Finally opened up the Anthem, all 18 screws! Replaced the tube with it's predecessor, a Tesla -- possibly a step back sonically from what I had had before. Happy now ;-) Although, the magic sort of disappears as the last screws are torqued on. I really suspect metal covers are detrimental to the sound of components.

Anyway, I never thought that one tube would distribute it's noise to all channels and tubes. I suspect it was the filament, whose circuit is basically common to all tubes. I wonder what the noise mechanism is??

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First I can't figure out how having a metal cover would make the sound better. The cover should help get rid of potential RMI or RFI. Do you think the open air vibrations create a better 3d effect, I just can't see it. Also are you saying a cryoed EH current production had more tube magic albeit noisy, than an old stock Tesla. That just sounds improbable to me. Obviously taste is subjective so it's entirely possible. Finaly how does the pin filament circuitry on one tube make other filaments noisy?? I am no EE so a simple explanation would be appreciated. Is it like, the noisy amplification from the first tube in a circuit just gets amplified and modified by all the subsequent circuit components?

They are definitely detrimental to sound. Basically the covers form a giant ground plane, being parallel to the printed circuit boards. Forms a tiny but physically large capacitor which rolls off the top end. replacing the cover with a plexi glass sheet, gives the dust protection but retains the "openness" of removing the cover. The effect is even greater is you replace the bottom cover. which lies closer to the boards.

No, I meant the new, not NOS, [EH's, not Tesla's] were at the time a sonic upgrade -- also, the Tesla's were in use some years which is another factor. It is plus one year now for the EH's, and I can't precisely compare now to back then. Yes, IMO metal covers ARE a sonic degradation to the sound. I don't live near a transmitter station, so RFI is not an issue. The EH's cost more than the Tesla's, current prices, and I do think there is some correlation between price and performance. Cryo'ed, well the tubes were fine when I got them.